In nuclear reactors, particularly in pressurized water nuclear reactors, the core of the reactor constituted by a set of prismatically shaped assemblies, usually of square section and disposed vertically, has a cooling liquid passing through it, usually water, in the longitudinal direction of the assemblies to cool the latter and transfer heat from the core to the steam generators.
In pressurized water reactors, the square section assemblies are disposed vertically and the pressurized cooling water itself circulates in the vertical direction in contact with the fuel assemblies constituted by a set of tubes containing the combined fissionable material in the form of a bundle.
The whole core is disposed in a casing skirt which is usually cylinder-shaped like the vessel which contains the core and its attached fittings immersed in the cooling liquid.
The set of assemblies constituting the core presents a section of irregular shape comprising steps which fit inside the section of the casing skirt, so that an annular fuel-less region is formed between the peripheral assemblies and the casing skirt.
To preserve the initial geometry of the core and prevent the coolant from circulating freely in this region and channel it to the core proper, the use of a partitioning assembly which exactly matches the contour of the core is known. Fixing this partitioning assembly poses problems of practical implementation connected with the fact that there are very strict requirements for dimensional accuracy for this assembly and that the conditions prevailing in the reactor vessel in service (pressure, temperature and radiation) make it necessary to take account of the possibility of the structure deforming in various ways in service.
Reactor partitioning apparatuses of the prior art are usually formed in one piece and constituted by longitudinal partitioning plates fixed to transverse reinforcement plates termed shapers which are themselves fixed to the cylindrical core casing. Such one-piece structures are difficult to make, and positioning and fixing them inside the cylindrical skirt encasing the core is also a tricky operation. The unit constituted by the casing skirt and the partitioning apparatus must in fact provide a perfectly defined space for the assemblies constituting the reactor core and withstand deformations in service inside the reactor vessel.
In addition, the devices for fixing the elements constituting this complex structure, whether constituted by screws or lugs associated with welds, are extremely sensitive to the mechanical stresses and to the neutron fluxes which the structure must withstand inside the vessel of the reactor when the latter is in service.
The object of the invention is therefore an apparatus for partitioning off the core of a nuclear reactor with removable elements, the core of the reactor being constituted by prismatic assemblies juxtaposed inside a casing laterally bounding a space surrounding the core in which a cooling liquid circulates in the longitudinal direction of the assemblies, a lower support plate and an upper support plate also being disposed on either side of the core in the longitudinal direction. The partitioning apparatus has a structure which facilitates its construction and positioning in the casing skirt of the core and assures that its deformations under the effect of conditions prevailing in the core of the reactor in service are compatible with the functions of the partitioning apparatus and allow deterioration of the connecting elements of the structure to be avoided.
To achieve this object, the partitioning apparatus according to the invention is constituted by a set of boxes, each comprising at least two plates rigidly assembled at right angles, disposed in the longitudinal direction of the assemblies, and a plurality of transverse plates perpendicularly disposed to the longitudinal plates and rigidly assembled to the plates, these boxes being disposed removably side by side inside the core casing and bearing thereon, so that the faces of the longitudinal plates directed towards the reactor core serve as bearing faces for the assemblies disposed at the periphery of the core, each box being fixed to at least one of the upper or lower support plates, at at least one of its ends, by a flexible coupling allowing longitudinal displacements of the box.